1. Field of the Invention
This invention relates generally to optical waveguide devices, and particularly to an optical waveguide having an array of single mode fiber cores positioned in a common cladding such that the optical channel has a particularly wide bandwidth and a large light-carrying capacity.
2. Description of the Prior Art
Optical waveguides have been known for many years and, with the advent of low loss glasses, devices incorporating optical waveguides have been employed in ever-increasing numbers, in many different fields, such as communications and monitors. An optical waveguide typically consists of a dielectric core fabricated from a glass, or the like, having a certain index of refraction, and this core is surrounded by a second material, also normally glass or the like, having a lower refractive index. This surrounding material is generally known as the cladding. A beam of light is guided by this composite structure so long as the refractive index of the material comprising the core exceeds the refractive index of the material forming the cladding. A light beam within the core is guided generally along the core axis by total internal reflection at the boundary between the core and cladding.
A number of different designs for optical waveguides have been proposed including the multimode step index profile, the single mode profile, and the multimode graded index profile. In the multimode step index design, the core typically has a relatively large diameter and is fabricated from a homogenous material having a high refractive index. The cladding is also fabricated from a homogenous material which has a sufficiently lower refractive index such that the difference between the refractive indices of the core material and cladding material is typically of the order of 10.sup.-2 to 10.sup.-1. Because the diameter of the core is large with respect to the wavelength of light, i.e. typically on the order of 50-500 .mu.m, many modes exist in the waveguide when illuminated. Because each mode as it propagates through the waveguide will experience a slightly different delay, a condition known as modal dispersion occurs which increases the duration of a light pulse propagating through the waveguide and leads to a loss in high frequency information in the light signal.
In the single mode optical waveguide, the diameter of the core is typically less than 10 .mu.m and the difference between the refractive indices of the core and the cladding is of the order of 10.sup.-3. As a result only the lowest order mode will be supported in the waveguide. Because of the small numerical aperture and small core diameter of the waveguide, only a small portion of the light emanating from an incoherent source will be coupled into the waveguide. In fact, with the typical light emitting diodes available at this time, the amount of energy coupled into the optical waveguide is on the order of 10.sup.-4.
The multimode graded index optical waveguide was developed to retain a high input coupling capability associated with a large numerical aperture and core diameter while at the same time attempting to minimize the modal dispersion losses associated with the propagation of a pulse having a number of modes, each of which propagates at a different velocity. The core of the waveguide is fabricated such that the core does not have a constant refractive index but decreases across the radius of the core with the axis of the core having the highest refractive index. Although there is considerably less modal dispersion in the graded index optical fiber when compared to the multimode step index optical fiber, there is still some dispersion because the gradient in index required for the axial rays is different from that required for the skewed rays. In addition, only a portion of the theoretical improvement in channel bandwidth has been realized because of manufacturing tolerances associated with the fabrication of a core having a core profile with a precisely graded refractive index.